Epoxy resins and epoxy resin compositions containing curing agents as essential components have high heat resistance and are excellent in terms of various properties such as moisture resistance and hence are widely used for, for example, semiconductor sealing materials, electronic components such as printed circuit hoards, the electronic component field, conductive adhesives such as conductive pastes, other adhesives, matrices for composite materials, coating materials, photoresist materials, and development materials.
In recent years further enhancement of properties represented by heat resistance, moisture resistance, and solder resistance has been demanded in such various applications, in particular, applications to advanced materials. Vehicle-mounted electronic devices that are particularly required to have high reliability and were mounted within cabins have come to be mounted within engine compartments having a higher temperature than cabins. In addition, reflowing treatment temperature has increased due to use of lead-free solders. Accordingly, there is an ever increasing demand for materials having excellent heat resistance.
When epoxy resin compositions are used as materials for printed wiring hoards, to impart fire retardancy to epoxy resin compositions, the compositions are mixed with fire retardants containing halogen such as bromine together with antimony compounds. However, with efforts in terms of environment and safety in recent years, there has been a strong demand for the development of a environmentally friendly and safe method for making compositions have fire retardancy without halogen fire retardants that may emit dioxins and without antimony compounds that may cause cancer. In addition, in the field of materials for printed wiring boards, use of halogen fire retardants causes degradation of reliability of printed wiring boards left to stand at a high temperature. Accordingly, halogen-free compositions are highly expected.
As for an epoxy resin composition that satisfies such required characteristics and has fire retardancy and high heat resistance, for example, Patent Literature 1 described below discloses a technique of using, as a curing agent for epoxy resins, a phosphorus-containing phenolic resin that is obtained as follows: 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (hereafter, abbreviated as “HCA”) is allowed to react with formaldehyde or acetone to provide a phosphorus compound having a hydroxy group and this phosphorus compound is allowed to react with a phenolic resin. However, in the production process of such a phosphorus-containing phenolic resin, reactivity between polyfunctional phenols and HCA and aldehydes low and hence reaction products between HCA and aldehydes remain as unreacted components in the resultant phenolic resin. Accordingly, although the cured product of the resin has high fire retardancy, the cured product is poor in a thermal decomposition property and cannot pass a thermal delamination test (hereafter, abbreviated as “T288 test”) that has been thought to be an important evaluation method for lead-free solder implementation in recent years. In addition, due to the above-mentioned low reactivity between the raw materials, the type of usable polyfunctional phenols is limited and the range of designing phosphorus-containing phenolic resins is considerably limited.
Patent Literature 2 described below discloses, as an intermediate phenolic compound of a phosphorus-containing epoxy resin, a compound obtained by allowing reaction products between HCA and hydroxybenzaldehyde to react with phenol.
However, as for this phenolic compound, reactivity between phenol and reaction products between HCA and hydroxybenzaldehyde is also insufficient and the degree of freedom with which the resin is designed is low. In addition, the finally obtained phenolic compound has a melting point of 200° C. or more and it is difficult to industrially produce this compound. Furthermore, the phenolic compound is a crystalline substance and has a poor dissolution property in organic solvents. Accordingly, the phenolic compound is poor in processability when being handled.
Patent Literature 3 described below discloses a fire-retardant epoxy resin composition in which a phosphorus-modified epoxy resin obtained by allowing a phenolic novolac epoxy resin or a cresol novolac epoxy resin to react with HCA is used as a basic resin and is mixed with a curing agent for an epoxy resin. However, to introduce phosphorus atoms into the structure of the epoxy resin, HCA is allowed to react with epoxy groups that are supposed to serve as cross-linking points. Accordingly, the epoxy resin composition described in Patent Literature 3 does not achieve a sufficiently high cross-linking density and the cured product has a low glass transition temperature. Thus, the epoxy resin composition is not usable for lead-free solder implementation.
As described above, as a method for imparting fire retardancy to a resin component, a technique of using HCA as a modifying agent for a phenolic resin or an epoxy resin is known. When a phosphorus atom is introduced into a phenolic structure by allowing a reaction product between HCA and an aldehyde or a ketone to react with the aromatic nucleus of the phenolic structure, the reaction product has low reactivity and hence the cured product of the resultant phosphorus-containing phenolic resin has insufficient heat resistance and does not exhibit properties for passing a thermal delamination test (hereafter, abbreviated as “T288 test”). In addition, since the reaction product between HCA and an aldehyde or a ketone has low reactivity, phenols that are usable in reaction with the reaction product are limited. Alternatively, when HCA is allowed to react with epoxy groups of an epoxy resin, the concentration of the epoxy groups decreases and hence a sufficiently high heat resistance cannot also be achieved.